| dc.description.abstract |
White light emitting diodes (WLEDs) have been investigated comprehensively due to their broad applications in lighting and display and the application of quantum dots (QDs) as novel down-converting materials in WLEDs has been also attracted lots of attentions because of their controllable emission wavelength, narrow band emission and high quantum yields (QYs) of QDs. The common method to prepare the WLEDs is mixing multi-colors of QDs, which is complex, time-consuming and has strong reabsorption. Therefore, in this study, we have demonstrated a facile hot-injection method to synthesize direct bi-color CdSe QDs (named as GRx, x = 1.5, 1.1 and 0.75, x value is associated with TOP-Se concentration) with dual-wavelengths against the reabsorption effect, and the GRx QDs have been applied to fabricate the QDs- and Y3Al5O12:Ce3+ (YAG:Ce)-based WLEDs.
This study includes three major parts. The first part focuses on the preparation and characterization of bi-color QDs. The results show that the GRx QDs consist of different emission intensities between green and red QDs through control of nucleation and growth rates. The excitation area ratio of green to red (Agex/Arex) of QDs can be significantly increased as the x value decreases, meaning that the energy of the green QDs absorbed by the red QD is reduced to improve the reabsorption effect. However, the QY of these GRx QDs is mainly affected by the degree of Se oxidation. The GR0.75 QDs have the lowest degree of Se oxidation, the highest QY value, the highest excitation area ratio, and the best stability after aging in hexane for 4 months.
The second part focuses on the application of different GRx QDs in blue LED. The results show that the GR0.75 QDs can strongly against the reabsorption effect and generate the white light successfully. When the weight ratio of UV resin in GR0.75 QDs increases from 0 to 45 wt. %, the color can be easily adjusted from warm to cold white light, and the operation time can be increased from 40 min to 18 hr. However, when the UV resin content is 30 wt. %, the Commission international de I’Eclairage (CIE) located at (0.35, 0.34) can be applied as backlight source, providing 89 and 126 % color gamut in NTSC and sRGB standards, respectively.
The third part focuses on the performance of YAG:Ce-based WLED when the GRx QDs have been added to YAG:Ce phosphor as additional components. The color of YAG:Ce-based WLEDs can be changed from warm to cold white light as the contents of GRx QDs increase. The best improvement in luminous efficacy is the addition of 3 wt. % of the GR1.5 QDs and the efficacy can be improved from 73 to 88 lm/W. On the other hand, the CRI can be promoted from 66 to 83 when the GR1.5 QDs content is 50 wt. %. Besides, the optimal condition to be applied in solid state lighting is mixing with 30 wt. % GR1.5 QDs. The CIE, CCT, CRI and luminous efficacy of device properties is (0.33, 0.34), 5482 K, 75 and 65 lm/W, respectively. The results show that these GRx QDs have a significant effect on luminous efficacy and color rendering index (CRI) of YAG:Ce phosphor. In addition, the long-term test shows that the addition of GR1.5 QDs into WLED can not only maintain the initial lighting color and CRI, but also improve the overall efficacy.
Based on the above results, the bi-color GRx QDs can be prepared by controlling the nucleation and growth rates. In QDs-WLED applications, the GR0.75 QDs exhibit the potential as bi-color nanophosphor to simplify the preparation procedure and decrease the reabsorption effect. In addition, the GRx QDs can be used as additional components to increase the luminous efficacy and CRI, especially GR1.5 QDs, compared to the original YAG:Ce-based WLED. | en_US |